1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device. Particularly, the present invention relates to a semiconductor integrated circuit device, which has a capacitor of a crown structure and a method for manufacturing the same.
2. Description of the Related Art
A MIS (Metal-Insulator-Semiconductor) structure is used for capacitors of DRAMs commercially available at present. In order to achieve high integration as the product generation progresses, the area of a memory cell is reduced and a plane area or a space allowed to form a capacitor is also reduced. However, reliable DRAM operation requires a sufficient capacitance of the capacitor.
As is generally well known, the capacitance of a capacitor is dependent on the area of an electrode and a dielectric constant of a dielectric layer. In order to enlarge the effective area of an electrode, a method of providing unevenness on the surface of a silicon lower electrode comes into practice. In order to improve the dielectric constant, a method of using materials with a high dielectric constant such as tantalum oxide for the dielectric layer comes into practice. Although higher integration is requested, it is difficult to secure a sufficient space for the unevenness on the surface of the lower electrode. Also, the improvement of the dielectric constant by using tantalum oxide has a limitation when the lower electrode is formed of silicon.
In case of the MIS structure of the lower electrode of silicon and the dielectric layer of tantalum oxide, a silicon oxide layer with a low dielectric constant is inevitably formed at the boundary between the silicon lower electrode and the tantalum oxide layer. To solve such a problem, it is under consideration to use the MIM structure which has a metal lower electrode. However, increase in leakage current occurs as a new problem in the case of the MIM structure. Especially, metals and metal compounds such as titanium nitride and tungsten, which are already used in manufacturing semiconductor devices, are easy to be oxidized. Therefore, it is difficult to reduce the leakage current between dielectric layer of tantalum oxide and the metal electrode.
In order to avoid this problem, it is effective to form lower electrode by the use of materials such as platinum, ruthenium, and iridium, which are relatively hard to be oxidized. Moreover, it is essential for manufacturing an actual 3-dimensional structure of capacitor that a film of such a material can be formed by using a CVD method from the viewpoint of sufficient coverage and is easy to process or work. Ruthenium is the most promising material which fulfills these requirements.
As schematically shown in FIG. 1, when a crown structure consisting of ruthenium is formed, the crown structure loses a support section during a wet etching so that the crown structure is broken or collapsed. As a result, the production yield remarkably decreases. This phenomenon is conspicuous specifically in case that the crown structure is vibrated in the etching liquid. Also, this phenomenon sometimes occurs in a heat-treat process when an insulating film is formed on the crown structure.
Also, when a ruthenium film is formed by the CVD in practice, unevenness of the film is conspicuous, so that a thin portion appears locally, and many voids exist within the film. Thus, the formed ruthenium film is weak in mechanical strength. When a lower electrode of the crown structure is formed in this condition, problems such as break and collapse of the structures occur, which makes it difficult to ensure the production yield.
FIG. 2A schematically shows an observation of the cross section of a ruthenium film by a transmission electron microscope immediately after a ruthenium film is formed by the CVD method. The observed sample was prepared by forming a ruthenium film (not shown) of 5-nm thickness as crystal seed by a sputtering method after a silicon oxide film 502 was formed on the surface of a silicon substrate 501 and by forming a ruthenium film 503 by the CVD method to have the thickness of 30 nm. The formed ruthenium film 503 appeared flat when the formed ruthenium film 503 was observed by a scanning electron microscope in the magnification of about 200,000 times. However, when the magnification was increased to about 4,000,000 times, it was observed by the transmission electron microscope that the ruthenium film 503 grew selectively in a pillar shape and was not a continuous film, as shown in FIG. 2A. There are many vacant spaces around each pillar. In this case, most of the pillars does not extend in a vertical direction and grew inclined so that the neighboring ones of the pillars contact each other at the upper portion.
When such a ruthenium film is heat-treated, the ruthenium film is fluidized to form a continuous film, as shown in FIG. 2B. The sufficiently continuous film can be formed at about 400° C. At this time, the vacant spaces between the pillars are taken into the continuous film through the heat-treatment and remain as voids 505. Also, the film thickness of the continuous film is not uniform and many thin portions exist even after the heat-treatment. In an extreme case, a portion with no ruthenium appears as a defect 506. In this way, the formed ruthenium film lacks the mechanical strength due to the existence of these voids within the film and the relatively thin portion, which causes break and collapse of the ruthenium crown structure.
In conjunction with the above description, a semiconductor memory device with a crown structure is described in Japanese Patent Application Laid Open (JP-P2000-150827A). In this conventional example, a lower electrode is formed of amorphous silicon or polysilicon, and a dielectric film is formed from an oxide film or a nitride film-oxide film. The use of a ruthenium film is not described.
Also, a semiconductor memory device with a crown structure is described in Japanese Patent Application Laid Open (JP-A-Heisei 11-274431). In this conventional reference, an electrode is formed of titanium nitride and a dielectric film is formed of tantalum oxide. Also, a ruthenium film is formed on a titanium nitride film in an example of this reference. In the structure of the example, a base metal electrode is used for the sake of oxidation resistance and unevenness is provided on the base metal electrode to enlarge the surface area, and a ruthenium film is provided on the electrode. To achieve the structure, the following processes are carried out: (1) unevenness is provided on the base metal electrode by a wet etching, and (2) a ruthenium film is grown by the CVD method.
However, it is difficult to control the processes because the unevenness is formed by applying a wet-etching method to the electrode itself. In an extreme case, penetrating portions are formed in some etching portions of the electrode because the etching is carried out at both sides of the electrode, resulting in the remarkable loss of the mechanical strength. In order to form unevenness with a large enough size to embody the effect while avoiding such problems, the electrode before the etching needs to have a sufficient thickness, e.g. a thickness more than 100 nm. In such a case, however, holes of the electrode disappear. Therefore, such a structure is unsuitable for a semiconductor integrated circuit in which a high integration is required.